This Week in Science

Science  11 May 2012:
Vol. 336, Issue 6082, pp. 643
  1. Earthquake Model Shakedown


    The Parkfield segment of the San Andreas Fault in California experiences Magnitude 6.0 earthquakes at a surprisingly regular interval—roughly every 22 years. This area is one of the most well studied fault segments in the world, yet computational models often struggle to integrate the wealth of observational data with theoretical predictions. Barbot et al. (p. 707; see the Perspective by Segall) constructed a dynamic model of a fault segment which, when integrated with previous observations, reproduces the behavior of the Parkfield segment over the entire earthquake cycle. Because the model is based on realistic fault physics, it not only explains the distribution of small earthquakes but also the recurrence interval of large earthquakes and the amount of geodetic strain accumulated postseismically. It also reveals how smaller earthquakes can influence this region's semiregular earthquake cycle.

  2. A New Dawn

    Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.

  3. Tunneling Through with a Light Touch


    Quantum tunneling underpins a host of essential techniques, such as scanning tunneling microscopy and quantum cascade lasers, as well as chemical reactions. The tunneling particles are normally electrons, and control of the tunneling process has generally been by electric fields. By coupling tunneling electrons with cavity photons trapped inside a semiconductor microcavity, Cristofolini et al. (p. 704, published online 5 April; see the Perspective by Szymańska) produced mixed states that then allowed direct optical control of the tunneling process. Such an optical-based approach to manipulating and controlling the tunneling process may find applications in quantum information science.

  4. By the Numbers

    As carbon dioxide is exchanged between the atmosphere, the oceans, and the terrestrial biosphere, its carbon isotopic composition is modified by various processes involved in its transfer between the different reservoirs. The carbon isotopic composition of the carbon dioxide contained in bubbles of air trapped in ice cores thus provides a record of the processes that regulated the composition of the atmosphere in the past. Using data from three Antarctic ice cores, Schmitt et al. (p. 711, published online 29 March; see the Perspective by Brook) present a record of the carbon isotopic makeup of atmospheric CO2 for the past 24,000 years. The findings reveal the dominant role of the oceans during the early part of the deglaciation and the effects of the regrowth of the terrestrial biosphere later in the deglacial transition. Before the deglaciation, during the Last Glacial Maximum, the carbon cycle was essentially at equilibrium.

  5. Mayan Astronomy

    Mayan Codices are books written on bark in the few centuries before Columbus landed. Several record detailed hieroglyphic calculations of lunar and planetary motions and their relation to the Mayan calendar. Their predecessors have been unclear. Saturno et al. (p. 714) now describe a room in a Mayan complex in Guatemala dating to several centuries before the Codices that seems to have similar calculations on two of its walls. The east wall contains lunar calculations; the calculations on the north wall are more enigmatic, but may relate to Mars, Mercury, and/or Venus.

  6. Osteoarthritis and Kartogenin

    Osteoarthritis is characterized by progressive breakdown of articular cartilage and affects over 25 million people in the United States. Mesenchymal stem cells (MSCs), which reside in healthy and diseased joints, are multipotent adult stem cells that are able to differentiate into a variety of cell types. Johnson et al. (p. 717, published online 5 April) identified a small molecule, kartogenin, which was able to induce MSCs to differentiate into chondrocytes in vitro. When administered locally, kartogenin was efficacious in two animal models of osteoarthritis.

  7. The Right Move

    During development, epithelial tissues deform to give rise to functional tissues and organs. How gene expression controls local cell mechanical properties to drive tissue deformation remains poorly understood. Bosveld et al. (p. 724, published online 12 April) have uncovered how the conserved Fat/Dachsous/Four-jointed signaling pathway controls local mechanical cell properties to generate global tissue contraction in Drosophila epithelial tissue.

  8. Dissecting DNA Repair

    Covalent modification of proteins can be a crucial regulatory event. Poly(ADP-ribose) polymerase 1 (PARP-1) (ADP, adenosine diphosphate) synthesizes poly(ADP-ribose), which is attached to and regulates proteins involved in DNA repair. Langelier et al. (p. 728; see the Perspective by Gagné et al.) use x-ray crystallography and biochemical analysis to demonstrate how PARP-1 detects DNA damage and how the interaction with DNA is coupled to poly(ADP-ribosyl)ation activity.

  9. Find Your Partner

    Gametes generally have haploid genomes, so that when they fuse during fertilization, they reconstitute a diploid organism. Meiosis is required to make haploid gametes, which involves a reduction division where two homologous chromosomes first pair and are then segregated away from each other. Working in the fission yeast Schizosaccharomyces pombe, Ding et al. (p. 732; see the Perspective by Dernburg) explored how homologous chromosomes recognized each other and found that a non-coding RNA locus, sme2, helps to drive the pairing of homologous chromosomes.

  10. Reaction-Diffusion Patterning


    Six decades ago, Alan Turing proposed the reaction-diffusion model to explain how complex patterns emerge during morphogenesis. In this model, an activator activates both itself and an inhibitor (the “reaction”), with the activator being less mobile than the inhibitor (“diffusion”). Many activator/inhibitor pairs have been described, but the role of differential mobility versus differential clearance of activators and inhibitors is often unclear. Now, Müller et al. (p. 721, published online 12 April) show that during zebrafish embryogenesis, the activator Nodal has lower diffusivity than the inhibitor Lefty, while both molecules are cleared at similar rates, which supports the idea that Nodal and Lefty form a classical reaction-diffusion system.

  11. Colorectal Cancer Signature

    The mutations and genome aberrations that characterize cancer result in often dramatically altered gene and protein expression patterns. It is these altered expression patterns that directly and indirectly drive progression of the disease. In human primary colorectal cancer cells, Akhtar-Zaidi et al. (p. 736, published online 12 April) analyzed the pattern of epigenetically modified chromatin at “enhancer” sequences that are known to be critical in the control of gene expression. An epigenetic enhancer signature was defined that was specifically associated with colorectal cancer cells.

  12. Exponential Growth Effects

    Humans are an extraordinarily successful species, as measured by our large population size—approximately 7 billion—much of which can be put down to recent explosive growth. Leveraging human genomic data, Keinan and Clark (p. 740) examined the effects of population growth on our ability to detect rare genetic variants, those hypothesized to be most likely associated with disease. It appears that rapid recent growth increases the load of rare variants and is likely to play an important role in the individual genetic burden of complex disease risk.

  13. Silence, Please

    Polycomb group (PcG) proteins play pivotal roles in epigenetic gene control, development, and disease. PcG proteins can control cell cycle progression, but the underlying mechanisms remain unclear. PcG proteins are thought to silence transcription of target genes through modulating chromatin structure. Mohd-Sarip et al. (p. 744, published online 5 April) uncovered an unanticipated transcription-independent function for the canonical PcG protein “Posterior sex combs” in cell cycle control, through its direct regulation of cyclin B destruction.

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